Heroin use and the high fatality associated with heroin intoxication is once again on the rise in many societies. Treatment strategies for heroin abuse have traditionally targeted mu-opioid receptors (muORs) since heroin metabolites, e.g., morphine, selectively bind to this receptor subtype. In recent years evidence has accumulated to suggest that heroin abuse might also impair kappa-opioid receptor (kappaOR) function as a consequence of the disturbance to the normal homeostatic balance existing between the kappaOR and muOR opiate systems. kappaORs are the primary pharmacological target of dynorphyn (DYN) peptides and produce a number of behavioral and neurochemical effects that oppose those of muORs. The psychopathological disturbances including dysphoria evident in heroin abusers might result from an increase in activity of the DYN/kappaOR system known to regulate negative mood states. Very limited information currently exists as to what extent and where alterations to the DYN/kappaOR are manifested in the brains of heroin abusers. A likely site for DYN/kappa dysfunction related to heroin abuse is in the mesolimbic system, the proposed neuroanatomical substrate for reward and emotional regulation. It is the goal of this project to determine the neural actions of heroin on the DYN/kappa system, at the level of gene expression, peptide production, and receptor function by (1) postmortem analyses of discrete mesolimbic neuronal populations in human heroin addicts and (2) utilization of an experimental rat addiction model to characterize the time course of these changes. In situ hybridization histochemistry will be used to measure mRNA expression levels, western blot analyses to detect the translated proDYN peptide, radioimmunoassay to measure post-translational DYN A and B peptides, ligand binding autoradiography to assess opioid receptor binding sites, and guanosine triphosphate (GTP)-gammaS-stimulated binding to determine the efficacy of kappaOR stimulation as a consequence of the repeated use of heroin. The multilevel approach to the study will ensure a greater understanding of the DYN/kappaOR organization and regulation. By characterizing the DYN/kappa-related mesolimbic circuits in human heroin users, it might also be possible to develop even more effective therapeutic interventions for opiate abuse.